Directional couplers having directivity enhancing means



vApril 21, 1970I HcpuLTER "3,508,170

DIRECTIONALVCOUPLERS HAVING DIvRECTIVIvTY ENHANCING MEANS Original Filed July 2'?. 1965 2 Sheets-Sheet 1 lNvENroR HOWARD C. POULT'ER ATTORNEY April 21, 1970 H. c. POULTER DIRECTIONAL COUPLERS HAVING DIRCTIVITY ENHANCIG MEANSl 2 Sheets-Sheet 2 Original Filed July 27, `1965 AAI igure 3 rml-16 wie xgure 4 lgure 7 INVENTOR HOWARD C. POULTER BY @.Q.

ATTORNEY United States Patent O 3,508,170 DIRECTIONAL COUPLERS HAVING DIRECTIVITY ENHANCING MEANS Howard C. Poulter, Palo Alto, Calif., assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California Continuation of application Ser. No. 475,165, July 27, 1965. This application Sept. 27, 1968, Ser. No. 768,609 Int. Cl. H011) /14, 3/08 U.S. Cl. 333- 13 Claims ABSTRACT 0F THE DISCLOSURE CROSS-REFERENCE TO RELATED APPLICATION This is a continuation application of patent application Ser. No. 475,165, now abandoned, entitled Directional Couplers and filed on July 27, 1965, by Howard C. Poulter.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to directional couplers, and particularly to high directivity TEM mode couplers.

A TEM mode coupler comprises a pair of adjacent transmission lines positioned in transverse electric and magnetic field coupling proximity. Insofar as these transmission lines are parallel and positioned in a medium of uniform permittivity, energy is coupled from one transmission line to the other in an inherently directional manner, because the electric and magnetic field couplings cancel each other in one direction. Typically, however, these adjacent transmission lines are not parallel along the entire lengths thereof, but diverge at some point so that, for example, one of the transmission lines can be connected to a load or an auxiliary output connector. Wherever the adjacent transmission lines diverge, a nondirectional signal is produced in coupling energy from one transmission line to the other because the magnetic field coupling is decreased more than the electric field coupling by the cosine of the angle of divergence between the transmission lines. This nondirectional signal, which can seriously impair the directivity of a TEM mode coupler, may also be produced when adjacent and parallel transmission lines are positioned in a medium of non-uniform permittivity.

One method of reducing the nondirectional signal commonly produced in TEM mode couplers and thereby increasing the directivity of these couplers is to include compensating means within the coupler for altering the magnetic field and/ or the electric field coupling between the diverging portions of the adjacent transmission lines. Various couplers using this method are disclosed and claimed in Harmons co-pending patent application filed Aug. l2, 1963, and issued Aug. 31, 1965 as U.S. Patent 3,204,206 entitled High Directivity TEM Mode Coupler, and in Woolleys co-pending patent application filed Feb. 2, 1965, and issued Apr. 18, 1967, as U.S. Patent 3,315,- 182 entitled Directional Coupler Having Directivity Improving Means Situated Near End of Coupling Region.

3,58,l Patented Apr. 21, 1970 It is the general object of this invention to provide a high directivity TEM mode coupler including a pair of transmission lines having parallel portions and means positioned near at least one of these parallel portions for decreasing the nondirectional signal produced when energy is coupled between the transmission lines.

In accordance with the illustrated embodiment of this invention, these means comprise one or more dielectric orv conductive elements, for an air-filled TEM mode coupler, and a cavity or a conductive element for a plasticfilled TEM mode coupler.

Other and incidental objects of this invention will become apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 comprises a cutaway perspective View of an air-filled, coaxial coupler including a dielectric element between the parallel portions of the primary and auxiliary conductors according to one embodiment of this invention;

FIGURE 2 is a schematic diagram of the TEM mode coupler of FIGURE l;

FIGURE 3 is a sectional top View of the air-filled coaxial coupler of FIGURE 1 including a distributed compensation structure according to another embodiment of this invention;

FIGURE 4 is a sectional end view of the air-filled coaxial coupler of F-IGURE 3 taken along the line A-A;

FIGURE 5 is a sectional side view of a TEM mode coupler including a distributed compensation structure according to still another embodiment of this invention;

FIGURE 6 is a sectional end View of a plastic-filled strip line coupler including a cavity in the plastic medium between the parallel portions of the primary and auxiliary conductors according to this invention; and

FIGURE 7 is a sectional side view of the plastic-filled strip line :coupler of FIGURE 6 taken along the line B-B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGURE l, there is show an airlled coaxial coupler having primary and auxiliary conductors 10 and 12. These primary and auxiliary conductors 10 and 12 are positioned adjacent to one another in electric and magnetic field coupling proximity within an outer conductor 14 including a pair of parallel ground planes. Primary conductor 10 is connected between and supported by a pair of coaxial connectors 16 mounted in opposite ends of the outer conductor 14. The central portions of the primary and auxiliary conductors 10 and 12 are parallel, but one end portion of auxiliary conductor 12 diverges to one side of the outer conductor 14, where it is connected to and supported by another coaxial connector 16 mounted in the outer conductor. Similarly, the other end portion of the auxiliary conductor 12 diverges towards the same side of the outer conductor 14, but is connected to and supported by a load 18 (shown in FIGURE 3) which terminates on one of the ground planes of the outer conductor. A dielectric compensating element 20 is positioned within the outer conductor 14 on the inner surface of one of the ground planes in the electric and magnetic field coupling region between the parallel central portions of the primary and auxiliary conductors 10 and 12. Normally this dielectric element should not protrude into the region between the adjacent surfaces of the primary and auxiliary conductors 10 and 12. The dielectric compensating element 20 alters the ratio of the magnetic to the electric eld strengths so as to reduce the nondirectioanl signal that is produced in couplin g electromagnetic wave energy from one conductor to the other because of the diverging portions thereof and thereby improves the directivity of the TEM inode coupler.

The manner in which the dielectric element 20 reduces the nondirectional signal and increases the directivity of the TEM mode coupler may be explained with the aid of the schematic diagram of the coupler shown in FIG- URE 2. Considering first the parallel central portions of the primary and auxiliary conductors and 12 which are positioned in a medium of uniform permittivity, as represented in region I of the schematic diagram, the ratio of the mutual capacitance to the mutual inductance equals a constant for which electromagnetic wave energy is coupled directionally'between the conductors. However for the diverging portions of the primary and auxiliary conductors 10 and 12 which are positioned in the medium of uniform permittivity, as represented in region II of the schematic diagram, the ratio of the mutual capacitance to the mutual inductance equals another constant, so that electromagnetic wave energy is coupled nondirectionally between the conductors. As indicated in region II of the schematic diagram, the ratio of the mutual capacitance to the mutual inductance is altered because of capacitive unbalance ACm which is caused by the divergence of the primary and auxiliary conductors 10 and 12. Addition of the dielectric compensating element 20 in the'magnetic and electric field coupling region between the parallel central portions of the primary and auxiliary conductors 10 and 12 has the effect of placing the conductors in a region of nonuniform permittivity causing a negative a capacitive unbalance -ACm between the conductors as indicated in region III of the schematic diagram. This negative capacitive unbalance -ACm substantially cancels the effect of the capacitive unbalance ACm added by the diverging portions of the primary and auxiliary conductors 10 and 12 as indicated by the schematic diagram and the following equation:

Vmondireenonaly=K V1 ACm) e L eJLZol Referring to the sectional views of FIGURES 3 and 4, the air-filled coaxial coupler of FIGURE 1 is shown with a dielectric compensating element 22, `which extends adjacent to the full length of the parallel central portions of the primary and auxiliary conductors 10 and 12. This dielectric compensating element 22 has, for example, a generally triangular cross-section, and is positioned on the inner surface of one of the ground planes of the outer conductor `14 so that it protrudes into the electric and magnetic field coupling region between the parallel central portions of the primary and auxiliary conductors 10 and 12, but not into the region between the adjacent surfaces thereof. `A similar dielectric compensating element 214 should be similarly positioned on the inner surface of the other' ground plane of the outer conductor 14, as shown in FIGURE 4, for best results. These distributed compensation elements 22 and 24 increase the directivity of the air-'filled coaxial coupler as discussed above and have a minimal effecten 'the'impedance of the TEM mode coupler, since they are symmetrically positioned with respect to the primary and auxiliary conductors 10 and 12 and extend along the full length of the parallel portions thereof. Since the parallel central portions of the primary and auxiliary conductors are of greater lengththan the diverging portions, this placement of the distributed com pensation elements 22 and 24 provides greater length of uniform compensation than `would be possible by placement'along the diverging portions.

' The directivity of the TEM mode coupler may similar.

ly be improved by positioning dielectric compensating elements on the inner surface of one of the ground yplanes of the outer lconductor 14, with one adjacent to the outer edge of the primary conductor 10 and the other adjacent to vthe outer'edge of the auxiliary conductor 12. This is indicated by the dielectric compensating elements 26 and 28 shown-in phantom in FIGURES 3 and 4.

The'directivity of a TEM mode coupler, whether airfilled or plastic-filled, may also be improved byincluding a conductive compensating element, such as the conductive 'comb 30,` shown: in the sectional Vsidemview of FIGURE 5,

within the outer conductor .14. This conductive comb 30 is positioned on the inner surface of one of the ground planes of the outer conductor 14 so that its teeth protrude intoA the electric and magnetic field coupling region be.- tvveen the parallel central portions of the primaryl and auxiliary conductors, `10 and 12. The conductive comb may extend alongl the full length of the parallel central portions of`the=primary and auxiliary conductors 10 and 12, or any portion thereof.` i v r' v Referring now to the sectional views of FIGURES 6 and 7 there is shownra plastic-f`1lled strip line coupler inl cluding a cavity 32 (several cavities instead of one might also be used)y located in the plastic material 34 in the magnetic and electric field coupling region betweentthe parallel portions of the primary and auxiliary conductors 10 and 12. This cavity might also extend along the entire length of these parallel portions as indicated bythe cavity 36 shown in phantom in the sectional side yView of FIGURE 7. In either case ythe cavity alters the ratio of the magnetic `to the electric field couplings in a manner toincrease thedirectivity of theplastic-lled, strip ,line coupler asdiscussed above. Y What is claimed is:

1. ATEM mode coupler comprising: A v I a first electromagnetic waveenergy transmission path including afirst line conductor anda reference conductor; Y v a second electromagnetic wave energy'transmissionvpath including a second line Iconductor and saidrefer-y ence conductor; said line conductorsvbeing spaced a `finite distance from one another and from said reference conductor and including parallel portions positioned in trans- 'verse electric and magnetic eld coupling proximity; Va dielectric medium filling therregion between a con-y i tinuousV portion of said reference conductor and the" lparallel portions of said line conductors; and a solid dielectric compensating element having a greater permittivity than said dielectric medium,' said ysolid dielectric compensating elementbeing positioned adjacent to a`t least one of the parallel portions oflsaid line conductor'sand ventirely within the region be'- tweenthec'ontinuous portion of said reference yconductor and the parallel portions 4of saidvline conductors to improve the directivity 'of said fcouplefr'. f 2. A TEM mode coupler as in claim 1 Whereinsaiddelectric compensating'element is supported on said reference conductor between the parallel portionsof said line conductors. 'f f 3. A TEM mode coupler as in *claim ,2 whereinfy said reference conductor comprises an outer concluy or t fhaving firs't 'and second parallel, 4'plane porti ns spaced afinite distance apar-t; said-line" conductorsar'e supportedy in a plane between' fandparallel to'tlle r-frst and-'second portionsf-of said outer conductor; v f" .said Azli'ele'ctricfmedium1 comprises air llingfthe space between/Ithe first and second portions :of said outer,

v.conductoiyand the parallel portions of isaidcline" conductors. "4. A TEM.I rlodepcouplerv as in claim 3' wherein-said dielectric compensating. elementis positioned adjacent to the parallel portions of saidline conductorsfalong substantially thefull length thereof.

Y, 5. ATEM vmode coupler as -in claim A4 ported on fthe second -portion of said outer'conductor between the .parallelportions of said line conductors y v o s including another dlelectrlc Jcompensating velement having a greater. permittivityv than said .dielectric mediumv and being supand within the region between the second portion of said outer conductor and the parallel portions of said line conductors, said other dielectric compensating element being positioned adjacent to the parallel portions of said line conductors along substantially the full length thereof.

6. A TEM mode coupler comprising:

a first electromagnetic wave energy transmission path including a first line conductor and a reference conductor;

a second electromagnetic wave energy transmission path including a second line conductor and said reference conductor;

said line conductors being spaced a finite distance from one another and from said reference conductor and including parallel portions positioned in transverse electric and magnetic lield coupling proximity;

a dielectric medium filling the region between said reference conductor and the parallel portions of said line conductors; and

a plurality of spaced, conductive protrusions supported on said reference conductor adjacent to at least one of the parallel portions of said line conductors and entirely within the region between said reference conductor and the parallel portions of said line conductors to improve the directivity of said coupler.

7. A TEM mode coupler as in claim 6 wherein said spaced, conductive protrusions are arranged in a row along said reference conductor between the parallel portions of said line cond-uctors.

8. A TEM mode coupler as in claim 7 wherein:

said reference conductor comprises an outer conductor having first and second parallel, plane portions spaced a finite distance apart;

said line conductors are supported in a plane between and parallel to the tirst and second portions of said outer conductor;

said dielectric medium comprises air iilling the space between the lirst and second portions of said outer conductor; and

said spaced, conductive protrusions are supported on the iirst portion of said outer conductor and entirely within the region between the iirst portion of said outer conductor and the parallel portions of said line conductors.

9. A TEM mode coupler as in claim 8 wherein said spaced, conductive protrusions are positioned adjacent to the parallel portions of said line conductors along substantially the full length thereof.

10. A TEM mode coupler as in claim 9 including another purality of spaced, conductive protrusions supported on the second portion of said outer conductor between the parallel portions of said line conductors and within the region between the second portion of said outer conductor and the parallel portions of said line conductors, said other plurality of spaced, conductive protrusions being arranged in a row along the second portion of said outer conductor and being positioned adjacent to the parallel portions of said line conductors along substantially the full length thereof.

11. A TEM mode coupler comprising:

a first electromagnetic wave energy transmission path including a first line conductor and a reference conductor;

a second electromagnetic wave energy transmission path including a second line conductor and said reference conductor;

said line conductors being spaced a finite distance from one another and from said reference conductor and including parallel portions positioned in transverse electric and magnetic field coupling proximity;

a dielectric medium having a permittivity greater than one, said dielectric medium iilling the coupling region between the parallel portions of said line conductors and filling the region between said reference conductor and the parallel portions of said line conductors; and

a cavity in said dielectric medium, said cavity intersecting a plane passing through the parallel portions of said line conductors and being positioned in the coupling region directly between the parallel portions of said line conductors to improve the directivity of said coupler.

12. A TEM mode coupler as in claim 11 wherein:

said reference conductor comprises an outer conductor having rst and second parallel, plane portions spaced a finite distance apart;

said line conductors are supported in a plane between and parallel to the first and second portions of said outer conductor;

said dielectric medium iills the space between the first and second portions of said outer conductor; and

said cavity is positioned between the rst and second portions of said outer conductor and entirely within said dielectric medium.

13. A TEM mode coupler as in claim 12 wherein said cavity is distributed along substantially the full length of the parallel portions of said line conductors.

References Cited UNITED STATES PATENTS 2,951,218 s/196o Arditi 333-24 x 3,204,206 s/1965 Harmon 333-10 3,315,182 4/1967 Woolley.

HERMAN KARL SAALBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner U.S. Cl. X.R. S33-84 

